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PRESYS
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Instruments
Manual PID adjustment
The manual adjustment of the PID parameters will be based on the Method of
ultimate sensitivity developed by Ziegler and Nichols. By this method, the optimum
PID adjustment will be that in which the process reaction curve shows consecutive
oscillations peaks with amplitudes at the rate of 1/4.
In order to apply this method, the following described steps should be carried
out:
i) Set the integrative rate (INT) and the derivative time (DER) to zero, i.e., leave the
Controllers under proportional action only.
ii) Set an arbitrary gain value (PROP) and record the controlled variable.
iii) Adjust the gain value (PROP), also called sensitivity, until reaching a limit value in
which the controlled variable will show a uniform oscillation, i.e., with constant
amplitudes (see figure 27). Values above that limit produce amplifying oscillations,
whereas gain values below that limit will dampen those oscillations. By ultimate
sensitivity (Ku) it is meant that limit gain and by ultimate period (Pu) it is meant the
period of constant oscillation caused by this sensitivity.
Compute the optimum PID parameter adjustment according to the table below:
Control action
P
P + I
P + D
P + I + D
DCY - 2057 Pressure Controller
Fig. 27 - Steady state oscillation
Gain
0.5 Ku
0.45 Ku
0.6 Ku
0.6 Ku
Integrative Rate
(INT)
-
1/(0.83 Pu*)
-
1/(0.5 Pu)
Derivative time
(DER)
-
-
0.125 Pu
0.125 Pu
* Pu - given in minutes
Maintenance
Page 45
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